Rabbit (Shope) fibroma virus is a poxvirus that causes rabbit (Shope) fibromatosis (cutaneous and sometimes metastatic, intenal fibromas) in cottontails Sylvilagus and in European rabbits Oryctolagus. Antigenically this virus is closely related to myxomatosis virus (a leporipoxvirus that causes myxomatosis), to the hare fibroma virus (the etiological agent of fibromas in hares Lepus), and to the squirrel fibroma virus (a leporipoxvirus that is the causal agent of squirrel fibromas). Shope fibroma virus is a tumor producing, transmissible agent that was first isolated from an Eastern cottontail rabbit (Sylvilagus floridanus) in the US in 1932. This virus is transmissible to European wild and domestic rabbits Oryctolagus cuniculus and to wild cottontail rabbits Sylvilagus, and produces single or multiple localized fibromas in the skin of these animals. Shope fibroma virus infections are relatively widespread in wild cottontail rabbits in the United States and Canada. It is regarded as a benign, self-limiting disease in the wildlife population. The virus may persist for several months within the cutaneous lesions, and mechanical transmission by arthropod vectors appears to be the primary means of spread of this virus (biting arthropods; fleas and mosquitoes can transmit the virus experimentally but the virus does not replicate in these vectors). Wild cottontail rabbits in the area are the likely reservoir host, with spread of the virus by insect vectors. In rare occasions, Shope fibromatosis has been diagnosed in commercial rabbitries. In naturally infected cottontail and in European rabbits, grossly there are firm, flattened tumors that occur on the legs and feet, sometimes with involvement of the muzzle, periorbital, and perineal regions [see image - discrete nodular fibromatous growth on the forepaw of a Sylvilagus (cottontail) rabbit naturally infected with rabbit (Shope) fibroma virus]. These subcutaneous tumors may be up to 7 cm in diameter, are usually freely movable, and may persist for several months. In young rabbits, metastases to internal organs such as abdominal viscera and bone marrow may also occur. Microscopically, this neoplasm consists of a localized fibroblast proliferation, fibroblasts characteristically fusiform to polygonal, and mononuclear and polymorphonuclear cell infiltration. In European domestic rabbits (Oryctolagus cuniculus), subcutaneous masses may vary from myxoid in type to typical fibromas. Large, intracytoplasmic, eosinophilic viral inclusion bodies may be present in reactive mesenchymal cells – a dense network of fusiform to polyhedral fibroblasts in the dermis with prominent intracytoplasmic inclusion bodies can be seen in these fibromatous neoplastic lesions – and also in epidermal cells overlying the tumors. The typical gross and histological appearance of these circumscribed fibrous masses should facilitate the differentiation of Shope fibroma from myxomatosis and also from the horny, pedunculated epidermal growths present usually on eyelids and ears in cases of rabbit (Shope) papillomatosis (which is caused by Shope cottontail rabbit papilloma virus). However, the myxoid forms of rabbit Shope fibromatosis sometimes seen histologically can be confused with lesions of myxomatosis in the skin. Considerable variation in microscopic appearance may occur.

References:

Percy DH & Barthold SW (2007). Pathology of laboratory rodents and rabbits http://books.google.com/books?id=-_tJNUoCqGoC&printsec=frontcover#v=onepage&q&f=false

The Merck Veterinary Manual Online Text (9 ed., 2005) – Rabbit (Shope) Fibroma Virus http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/171326.htm&hide=1

Michigan Wildlife Disease Manual (DNR-MI-USA) – rabbit fibromatosis (Shope fibroma) http://www.michigan.gov/dnr/0,1607,7-153-10370_12150_12220-27256–,00.html

Williams & Barker (2001) – Infectious diseases of wild mammals http://books.google.com/books?id=I62ZO_-4cjwC&printsec=frontcover&source=gbs_v2_summary_r&cad=0#v=onepage&q&f=false